Precise size-matching between guest polyoxometalates and host metal-organic frameworks enables enhanced photocatalytic water oxidation

The development of robust and efficient heterogeneous photocatalysts for water oxidation is a significant challenge in solar fuel production. Polyoxometalate-metal-organic framework composites (POM@MOFs) represent a promising platform, yet achieving optimal electronic interaction remains a key goal. We demonstrate that electronic communication between polyoxometalates (POMs) and metal-organic frameworks (MOFs) enhances charge transfer kinetics while suppressing electron-hole recombination, with maximum efficiency achieved through precise size-matching between MOF cavities and encapsulated POMs. This principle is illustrated by Co4@UiO-67 (C1), where encapsulation of Na₁₀[(PW₉O₃₄)₂Co₄(H₂O)₂] in UiO-67's cavities creates a leaching-proof composite. This confinement prevents aggregation, yielding excellent photocatalytic water oxidation performance (283 TONs)-surpassing prior Co4@MOF systems. Mechanistic insights from photoluminescence reveal efficient charge separation, while post-catalytic analysis confirms structural integrity and reusability. Overall, this work introduces a new paradigm in the design of POM@MOF composites, positioning C1 as a robust, recyclable, and highly active photocatalyst for heterogeneous water oxidation.